bcachefs: Fix incorrect IS_ERR_OR_NULL usage

Returning a positive integer instead of an error code causes error paths
to become very confused.

Closes: syzbot+c0360e8367d6d8d04a66@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/btree_node_scan.c

@@ -275,7 +275,7 @@ static int read_btree_nodes(struct find_btree_nodes *f)
 		w->ca		= ca;
 
 		t = kthread_run(read_btree_nodes_worker, w, "read_btree_nodes/%s", ca->name);
-		ret = IS_ERR_OR_NULL(t);
+		ret = PTR_ERR_OR_ZERO(t);
 		if (ret) {
 			percpu_ref_put(&ca->io_ref);
 			closure_put(&cl);